752 CONDENSATION" OF SLATE ROCKS. [Ch. XXXVI. 



When it was afterwards rubbed to a flat surface perpendicular to the 

 pressure and in the line of elongation, or in a plane corresponding 

 to that of the dip of cleavage, the particles were found to have be- 

 come arranged in the same manner as in natural slates, and the 

 mass admitted of easy fracture into thin flat pieces in the plane 

 alluded to, whereas it would not yield in that perpendicular to the 

 cleavage.* 



Dr. Tyndall, when commenting in 1856 on Mr. Sorby' s experi- 

 ments, observed that pressure alone is sufficient to produce cleavage, 

 and that the intervention of plates of mica or scales of oxide of iron, 

 or any other substances having flat surfaces, is quite unnecessary. In 

 proof of this he showed experimentally that a mass of " pure white 

 wax after having been submitted to great pressure, exhibited a cleav- 

 age more clean than that of any slate-rock, splitting into laminae of 

 surpassing tenuity." f He remarks that every mass of clay or mud 

 is divided and subdivided by surfaces among which the cohesion is 

 comparatively small. On being subjected to pressure, such masses 

 yield and spread out in the direction of least resistance, small nodules 

 become converted into laminse separated from each other by surfaces 

 of weak cohesion, and the result is that the mass cleaves at right 

 angles to the line in which the pressure is exerted. The experiments 

 of Mr. Sorby in reference to the manner in which scales of mica and 

 oxide of iron arrange themselves in soft pipe-clay under compression 

 have been supposed to lend countenance to the opinion that the lami- 

 nation of basalt and trachyte, and even of some kinds of gneiss, and 

 the grain of certain granites, may all have been determined by a 

 mechanical cause, a movement having taken place after the develop- 

 ment of crystals in the pasty mass. 



Mr. Scrope, in his description of the Ponza Islands, ascribed the 

 zoned structure of the Hungarian perlite (a semi-vitreous trachyte) to 

 its having subsided, in obedience to the impulse of its own gravity, 

 down a slightly inclined plane, while possessed of an imperfect fluidity. 

 In the Islands of Ponza and Palmarolo, the direction of the zones is 

 more frequently vertical than horizontal, because the mass was im- 

 pelled from below upwards." J In like manner, Mr. Darwin attributes 

 the lamination and fissile structure of volcanic rocks of the trachytic 

 series, including some obsidians in Ascension, Mexico, and elsewhere, 

 to their having moved when liquid in the direction of the laminae. 

 The zones consist sometimes of layers of air-cells drawn out and 

 lengthened in the supposed direction of the moving mass. This 

 division into parallel zones, thus caused by the stretching of a pasty 

 mass as it flowed slowly onwards, he compares to the zoned or 

 ribboned structure of ice, which Professor James Forbes has endeav 



* Sorby, as cited above, p. 750, note. 



f Tyndall, View of the Cleavage of Crystals and Slate Rocks. 



% Geol. Trans., Second Series, vol. ii. p. 227. 



